rs601338 is found on chromosome 19 in the alpha(1,2)-fucosyltransferase FUT2 gene. The wild-type rs601338(G) encodes the "secretor" (Se) allele, while rs601338(A) encodes the "non-secretor" (se) allele.

A study of 115 Swedish adults concluded that rs601338(A;A) homozygotes have genetic immunity to infection by the Norwalk norovirus, a major (and contagious) cause of acute gastroenteritis worldwide among adults. [PMID 12692541] [PMID 16306606] This illness is also known as "cruise ship gastroenteritis."

Being a non-secretor may have other consequences, such as greater susceptibility to infection by influenza viruses and by some types of bacteria. 23andMe discusses these topics.

In some non-Caucasian populations, a different SNP is responsible for non-secretor phenotypes. For example, although the Se allele is absent in Japanese, 15% are non-secretors based on being homozygous for the non-secretor "sej" allele of SNP rs1047781. [PMID 8621666]

It also appears to affect the composition of the microbiome [1], with secretors apparently having higher levels of Bifidobacteria.